It is really difficult to say why you are experiencing the symptoms you mention. Did you have any other testing done to look for an underlying problem ie. blood work to evaluate hormone levels or electrolyte levels?
PAC's (Premature Atrial Contractions) are typically known to be a benign ectopic beat originating from the atria (upper 2 chambers of the heart) There are many things that can aggravate this condition. Nicotine, caffeine, alcohol, stimulants of any kind, stress, exercise, anxiety etc. There are many more, but those mentioned are the more common triggers.
Is there any significant past medical history?
How do you know you black out for 4 seconds? How fast are thinking fast is? Before you are ready to chalk it up to serious cardiac problems, be sure you are well hydrated, without caffeine on board, and well rested. There are MANY things that can lead to some of your symptoms and nearly all can be related to anxiety when you get to researching what it COULD be. Many people here have some serious heart problems and while we may give some info and some advice, it may be well beyond your needs. Check with your md. Over 100 is tachycardia, but that is really only if it is at rest. It should go up with exercise. It will go up with dehydration, stress, fever.... it is made to adjust to changing stimuli all the time. If your rate is over 200, you can expect to feel all that. If it is 120, not necessarily. Good luck but don't stress too much about it before it is necessary. You will find that lots of people find PVCs very annoying but not devastating.